In a wireless communication system, a transmitter side transmits data over a radio frequency (RF) channel, such that signal distortion may occur during data transmission. In order to allow a receiver side to normally decode the distorted signal, the receiver side has to recognize channel information and needs to correct the signal distortion on the basis of the recognized channel information. In order to allow the receiver side to recognize information of the transmission (Tx) channel, a signal known to both the receiver and the transmitter side needs to be transmitted, the receiver side recognizes the degree of distortion of the transmission (Tx) signal when receiving the signal over the transmission (Tx) channel, such that it can recognize channel information on the basis of the recognized degree of distortion. In this case, the transmission (Tx) signal known to both the receiver and the transmitter side is hereinafter referred to as a pilot signal or a reference signal.
Recently, in most mobile communication systems, when data or packets are transmitted, a method for improving data transmission/reception efficiency using a multiple input multiple output (MIMO) communication system including multiple transmission antennas and multiple reception antennas has been used. The MIMO communication system assigns an independent channel to each transmission (Tx) antenna in such a manner that it transmits an independent reference signal to each transmission (Tx) antenna, such that the MIMO communication system can recognize a channel condition between transmission (Tx) and reception (Rx) antennas.
In a general communication system, reference signals (RSs) may be largely divided into two RSs according to functions thereof: a common RS for acquiring channel information and an RS used for data demodulation. The former is used for enabling a User Equipment (UE) to acquire downlink channel information. Therefore, the common RS should be transmitted in a wideband, and even a UE which does not receive downlink data in a specific subframe can receive and measure this RS. In addition, the common RS is also used for measurement for handover or the like.
The latter is an RS which is sent together with resources when a base station (eNB) sends downlink data. The UE may receive this RS so as to perform channel estimation and data demodulation. Therefore, this RS for data demodulation is transmitted in a region in which data is transmitted.
Meanwhile, in recent times, communication systems have a tendency to gradually increase a service frequency band as well as to gradually reduce a radius of each cell, such that the recent communication systems can implement high-speed communication and accommodate much more traffic. As a result, the conventional centralized cellular wireless network system has problems in that it cannot adapt to future changes. That is, the position of a base station (BS) is fixed, which deteriorates the flexibility of wireless link construction. As a result, the aforementioned centralized cellular wireless network system has difficulty in providing an efficient communication service in a wireless environment in which traffic distribution or the amount of requested traffic is dramatically changed.
In order to solve the aforementioned problems, a multi-hop relay system has been proposed. The multi-hop relay system enlarges a cell service region by partially covering a dead zone generated in a cell region, increases system capacity, and applies a relay to an initial situation having a small number of service requests, resulting in reduction in initial installation costs.
FIG. 1 is a conceptual diagram illustrating a relay backhaul link and a relay access link for use in a wireless communication system.
Referring to FIG. 1, a relay node (RN) for use in a long term evolution-advanced (LTE-A) system performs forwarding of a link between a base station (BS) and a UE, and two links having different attributes are applied to an uplink carrier frequency band and a downlink carrier frequency band. A link between a BS and an RN is defined as a backhaul link. The backhaul link scheme may be classified into a backhaul downlink scheme and a backhaul uplink scheme. The backhaul downlink scheme transmits data using downlink resources according to a frequency division duplex (FDD) or time division duplex (TDD) scheme. The backhaul uplink scheme transmits data using uplink resources according to the FDD or TDD scheme.
The relay node (RN) may receive information from the BS through relay backhaul downlink, or may transmit information to the BS through relay backhaul uplink. In addition, the relay node (RN) may transmit information to a UE through relay access downlink, or may receive information from the UE through relay access uplink.
The LTE-A system is being improved to increase communication performance of a UE by installing an RN therein. However, the LTE-A system has not yet disclosed a method for performing RS transmission/reception between a BS and an RN.